Cyanidation method



Aug. 18,- 1942. R. LORD CYANID'ATION METHOD Filed Oct. 18, 1940 lQOBEQTLona,

, INVENTOR.

A TTORNEY.

Patented Aug. 18, 1942 UNHTLED STATES FATE! OFFICE CYANIDATION METHODRobert Lord, Los Angeles, Calif., assignor to Southwestern EngineeringCompany, L05 Angeles, Calif., a corporation of California ApplicationOctober 18, 1940, Serial No. 361,659

7 Claims.

This invention is concerned with the cyanidation of ores carryingprecious metals, i. e., gold, silver, and metals of the platinum group,and is concerned particularly with the cyanidation of ores which are nothighly amenable to treatment by the conventional cyanide process.

One of the particular object of the invention is to provide acyanidation procedure for diflicultly processed ores, whereby a highertotal recovery of precious metals is secured. A further 2 object of theinvention is to provide a cyanidation process for difficultly processedores carrying precious metals, in which a high regeneration of cyanideis obtained, reducing to an unusually low level the cyanide consumptionper ton of ore treated.

A further object of the invention, in its preferred embodiment, is toprovide a cyanidation process for difficultly processed ores carryingprecious metals, in which the extraction procedj ure is carried out inthe presence of a caustic alkali reagent at a greater alkalinity thanhas heretofore been found useful in the art.

During the many years of development of the use of the cyanide processin the recovery of precious metals, numerous ores have been found whichshow a comparatively low recovery from the cyanide treatment; it is mypresent conclusion that these difficultly processed ores may becontaminated by the presence of compounds normally insoluble inconventional cyaniding so- According to a preferred practice of the in-Vention, I carry out the cyanidation of a gold or other preciousmetal-bearing ore in the presence of a solution containing a causticcompound of an alkali metal in a concentration much greater than thatusually employed where such a compound is used to provide protectivealkalinity, and preferably corresponding to between five and fortypounds of sodium hydroxide per ton of solution. I further preferablycarry out the cyanidation process at a temperature materially aboveordinary natural water temperature and preferably in the neighborhood ofthickener is carried to the agitator series shown as comprising threeagitators A1, A2, and A3.

The discharge from the third agitator-As is fed to the No. 2 thickenerT2, the underfiow from N0. 2 thickener is carried to the No. 3 thickenerT3, and the underflow from No. 3 thickener is carried to a filter, whichdischarges the ore tailings. The overflow from the No. 3 thickener isreturned to the No. 2 thickener. The overflow from the No 2 thickener iscarried to a tank 5 which is designated mill solution, from whichportions may be withdrawn from time to time through a line 6 to the goldtank 4. The mill solution from tank 5 is carried through a line 1 to astorage tank 8 and thence through a conduit 80. to the ball mills 2. Thesolution from the gold tank 4 is carried to precipitation at 9, and theeffluent from the precipitation step, hereinafter called barrensolution, is carried through conduit H to a junction box I2.

Caustic (i. e., an alkali-metal hydroxide) may be added to the circuitat the discharge of the classifiers 3 in any suitable manner, as througha conduit l3 communicating with tanks Ma and Mb wherein caustic may beformed, for example, by a hydrothermal reaction with sodium carbonateand lime, water being conveniently supplied to said tanks in the form ofbarren solution from line H, and heat being supplied through the agencyof steam or in any other suitable manner.

It will be appreciated that there will be some tendency for conversionof the free caustic to carbonate in th solutions undergoing treatment,due to the presence of atmospheric carbon dioxide, and I prefer toregenerate this carbonate into free caustic and return the solution tothe circuit in order to effect maximum economies in reagents. The barrensolution from the precipitation zone 9 is carried to the junction box l2barren solution is conveniently carried out in a thickener 24 throughthe addition of lime from a-lime tank 25, to which water may beconveniently supplied in the form of barren solution a described abovein the case of caustic tanks [4a and Mb. The regenerated barren solutionoverflowing from the thickener 24 through line 21 is carried to acaustic-regenerated barren solution storage tank 28 from which it may bewithdrawn to various portions of the circuit; for example, it may bedelivered through a line 29 joining the underflow line 3| from agitatorA1, or through line 32 communicating with the line 22 from thickener T2underfiow to thickener T3, or through line 30 to the line 8a leading toball mills 2. The barren solution which is in excess of the capacity ofthe caustic regeneration apparatus may be withdrawn through a line 33from the junction box l2 and admitted alternatively or concurrently tothe No. 2 thickener T2, N0. 3 thickener T3, or agitator A3, as throughlines 34, 35 and 36 respectively, or may be returned ahead of the No. 1thickener T1 as through conduit 37 indicated in dot-dash lines.

For the proper practice of the invention I maintain an amount of freecaustic in solution in the circuit which is at least greater than thatusually employed for protective alkalinity, and is not less than aconcentration corresponding to five pounds of sodium hydroxide per tonof solution (one-quarter of one per cent by weight of the solution), andpreferably carry the concentration in the neighborhood of thatcorresponding to ten pounds of sodium hydroxide per ton of solution(one-half of one per cent). Caustic concentrations as high as 40 poundsper ton (two per cent) or more, are useful, with some increase in totalrecovery of precious metal values. As an example of the increase inrecovery obtained according to the present process, one particular orehaving a gold content of approxmately 0.2 ounce per ton was laboratorytested by conventional cyanidation; an extraction of approximately 80per cent was obtained. Laboratory tests on the same sample using fromthirty to forty pounds of NaOH per ton of solu tion, at roomtemperature, and using ten pounds of NaOH per ton of solution atapproximately 90 F., showed an extraction of from 88 per cent to 90 percent.

According to the preferred practice of the invention I maintain thesolutions undergoing treatment at a temperature somewhat greater thanordinary water temperature and preferably at least in the neighborhoodof 90 F. At this temperature, a caustic concentration in theneighborhood of one-half of one per cent pounds per ton of solution) hasapproximately the same effectiveness as far as total precious metalrecovery is concerned as does a two per cent concentration solution attemperatures in the neighborhood of 60 F. Any suitable heating means maybe provided for supplying heat to the solution undergoing treatment,.atany desired locations in the process.

The caustic may be supplied as sodium hydroxide, or any other hydroxideof an alkali metal. For the purpose of simplicity, I have referred tothe alkalinity in terms of sodium hydroxide merely as illustrative ofthe degree of alkalinity produced in the solution.

I preferably maintain a cyanide concentration on the order of two andone-half pounds throughout the circuit, and it will be appreciated thatthis cyanide can be added at any position. In the above-describedpreferred embodiment of this invention, the high caustic concentrationis maintained throughout the agitator series A1, A2, and A3, whereinregeneration of cyanide takes place as the result of aeration, and ithas been found that this regeneration is effective to materiallyincrease the cyanide content. For example, in actual practice of thisinvention on plant scale, wherein the discharge from No. 1 thickener ranan average of 2.57 pounds of cyanide (as sodium cyanide) per ton ofsolution, the discharge from the No. 3 agitator ran 3.39 pounds ofcyanide per ton of solution as an average over the same period.

In its preferred application, the present invention contemplates the useof the high caustic concentration, together with cyanide, in theballmilling or other operation in which the final comminution of the oreis obtained, and it is believed that the principal proportion of thesolution efiect takes place in the comminution step and in thesubsequent classifiers. The process of the present invention isapplicable where conventional cyanidation is practiced, using theconventional forms of apparatus. It will be appreciated that freshcaustic may be supplied to the circuit instead of employing the causticregeneration step as herein specifically set forth, but a preferredpractice of the invention contemplates the continuous regeneration ofcaustic from the alkali carbonate compounds which tend to accumulatethroughout the circuit.

I claim:

1. The process of recovering precious metals from ores which comprisessubjecting such an ore to the solvent action of a cyanide solutioncontaining an alkali metal hydroxide in a concentration corresponding tobetween five and forty pounds of sodium hydroxide per ton of solution,and substantially recovering such precious metal from the resultingsolution.

2. The process of recovering precious metals from ores which comprisessubjecting such an ore to the solvent action of a cyanide solutioncontaining an alkali metal hydroxide in a concentration corresponding toat least five pounds of sodium hydroxide per ton of solution and at atemperature at least in the neighborhood of F., and subsequentlyrecovering such precious metal from the resulting solution.

3. The process of recovering precious metals from ores which comprisessubjecting such an ore to a comminuting operation and a subsequentcontact operation in the presence of a cyanide solution containing analkali metal hydroxide in a concentration corresponding to between fiveand forty pounds of sodium hydroxide per ton of solution, andsubsequently recovering such precious metal from the resulting solution.

4. The process for the recovery of precious metal from an ore containingthe same which comprises: subjecting such an ore in the form of a pulpto the solvent action of a cyanide solution containing an alkali metalhydroxide in a concentration corresponding to at least five pounds ofsodium hydroxide per ton of solution; subjecting such pulp to athickening operation; separating therefrom a solution containing theprincipal proportion of such precious metal; recovering such preciousmetal from such solution; treating at least a portion of the solutionseparated from the solids, at one point in the circuit, to regeneratealkali metal hydroxide from alkali metal carbonate formed in thesolution during the process; and returning such regenerated solution tothe circuit for use in the treatment of a further quantity of ore.

5. A process for the recovery of precious metal from an ore containingthe same which comprises: subjecting such an ore to contact with acyanide solution containing an alkali metal hydroxide in a concentrationcorresponding to between five and forty pounds of sodium hydroxide perton of solution, to dissolve precious metal from the ore; separatingundissolved solids from the solution containing dissolved preciousmetal; returning solution separated from the ore to contact with afurther quantity of such ore to establish a circuit; recovering preciousmetal from a portion of the solution separated from the solids; treatingat least a portion of the solution separated from the solids, at onepoint in the circuit; to regenerate alkali metal hydroxide from alkalimetal carbonate formed in the solution during the process; and aeratingthe solution while in contact with the ore and while containing aconcentration of alkali metal hydroxide within the above range, atanother point in the circuit, to cause regeneration of cyanide in thesolution.

6. A process for the recovery of precious metal from an ore containingthe same which comprises: subjecting such an ore to contact, in atreatment circuit, with a cyanide solution containing an alkali metalhydroxide in a concentration corresponding to between five and fortypounds of sodium hydroxide per ton of solution, to dissolve preciousmetal from the ore; separating a portion of the solution containingdissolved precious metal from a'thiokened product containing undissolvedsolids and solution; recovering precious metal from said separatedportion of the solution; treating said portion of the solution toregenerate alkali metal hydroxide from alkali metal carbonate formed inthe solution during the process; returning the so-regenerated solutionto the circuit; subjecting said thickened product to aeration to causeregeneration of cyanide therein; separating the solution from theundissolved solids of said areated thickened product, and returning saidseparated solution to the circuit.

7. A process for the recovery of precious metal from an ore containingthe same which comprises: subjecting such an ore to contact with acyanide solution containing an alkali metal hydroxide in a concentrationcorresponding to between five and forty pounds of sodium hydroxide perton of solution, and aerating at least a porton of the solution while incontact with the ore and while maintaining in the solution aconcentration of alkali metal hydroxide within the aforesaid range.

ROBERT LORD.

CERTIFICATE OF CORRECTION.

Patent No. 2,295 ,066. August 1 19m.

ROBERT LORD It is hereby certified-that error appears in the printedspecification of the above numbered patent requiring correction asfollows: Page 2, first column, line 58-59, for "approxmately" read--approximately--; and second column, line 57, claim 1, for"substantially" read subsequently--; page 5, second column, line 11;,claim 6, for "areated" read "aeratedline 21+, claim 7, for "porton" read--portion-; and that the said Letters Patent should be read with thiscorrection therein that the same may conform to the record of the casein the Patent Office. L

Signed and sealed this 15th day of October, A. D, 19L 2.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.

